1. Field of the Invention
The invention relates to the distribution of radio frequency signals using optical fiber links between a centrally located radar transmitter/receiver and a plurality of remotely located transmit/receive modules, each module being associated with an individual antenna element. Optical signal distribution, which maintains accurate phase relationships between distributed signals, has particular application to arrays requiring a low profile or reduced weight as in airborne applications.
2. Prior Art
The fiber optic distribution of RF signals is of advantage in airborne phased array radar systems. In a conventional arrangement, the RF signal modulates a locally generated optical carrier at the sending end of the distribution link. The modulated carrier is then coupled by an optical fiber to the receiving end of the link where the RF signal is recovered by an optical detector. For two-way communication, two optical sources, two means for modulating the sources, and two optical detectors are required. The optical fiber is of low loss, is immune to electromagnetic pulse (EMP) and exhibits low channel-to-channel cross coupling. Optical fibers are mechanically flexible, small in size, and light in weight, giving this method of signal transmission an advantage in airborne applications.
Present day phased array radar systems comprise a transmitter, receiver, beamformer, signal processor, display, power supplies, and an operator console which are centrally located and a large number of transmit/receive modules which are at some distance from the central location and in close association with the antenna elements making up the array. The active electronic device of such radar systems are solid state devices configured as discrete devices on printed circuit boards, integrated circuits, hybrid integrated circuits, monolithic microwave integrated circuits, also leading to small, light weight electronics ideal for airborne applications.
Proposed signal distribution systems for phased array radar systems employ laser or light emitting diodes as the modulated optical sources. During transmission, modulated light signals which are generated at the central location, are sent via optical fibers to the remote T/R modules. The T/R module includes detectors such as PIN diodes or photo diodes which recover the transmit signal from the optical carrier and RF amplifiers which amplify the transmit signal to a power level suitable for driving the antenna elements of the array.
The T/R modules also contain low noise amplifiers for amplifying the radar return, light sources for providing an optical carrier and a means for modulating the optical carrier with the receive signal. During reception, the optically formatted receive signal is conveyed by an optical fiber from the T/R modules to the central location where detection, beamforming and signal processing occur.
T/R modules are plentiful in a phased array radar system. The radar system typically requires one T/R module for each antenna element in the array. Since the antenna elements number in the hundreds or thousands, depending upon the complexity of the array, each radar system, depending on complexity, requires hundreds or thousands of T/R modules. Accordingly, any simplification of a module can have a major impact upon the cost and power requirements of the complete radar system.
The presently known optical distribution configuration for a phased array radar system requires optical sources at both the transmitter/receiver and at each T/R module. Attendant with each of these optical sources are temperature compensation and prime power systems.
Accordingly, any arrangement which reduces the complexity of the optical elements in the radar system or in particular avoids the need for an optical source in the T/R module, is of substantial value.
These and other objects of the invention are achieved in a radar apparatus comprising an array of M antenna elements, a radar transmitter, a source of an optical carrier, means to modulate the optical carrier by the transmitter signal and supply it to M separate optical paths, a radar receiver having a set of M optical inputs with demodulators for recovering M antenna returns from an optical carrier, and a set of M T/R modules each associated with an antenna element and each having a fiber optics input for connection to the transmitter and a fiber optics output port for connection to the receiver.
The apparatus further comprises a first set of M optical fibers for separately conveying the optical carrier bearing the transmitter signal (during the transmit period) to each T/R module, and a second set of M optical fibers for separately connecting the antenna signal from each antenna element, modulated on an optical carrier, to the receiver.
Each of the T/R modules further comprises a directional optical single pole double throw (SPDT) switch having a common port, a port selected during the receive period and a port selected during the transmit period. The optical carrier, modulated during the transmit period and unmodulated during the receive period is coupled to the common port of the switch.
An optical detector, coupled to the transmit port of the optical SPDT switch, recovers the transmitter signal from the optical carrier and a power amplifier amplifies the transmitter signal to a level suitable for application to an associated antenna element. A circulator couples the amplified transmitter signal to the associated antenna element during the transmit period, and couples the return from the associated antenna element to a low noise amplifier, which amplifies the return to a level suitable for subsequent processing during the receive period.
Finally an optical modulator, modulates the unmodulated optical carrier, available during the receive period from the STDT switch, by the amplified return. The optical carrier, modulated by the return is then coupled via the optical output port of the T/R module and one of the second set of optical fibers to the receiver.
The arrangement avoids the need for providing a separate laser for generating an optical carrier within the individual T/R modules for receive, and simplifies each module.
The reduction in the required number of lasers in a given system enhances the reliability of that system. Each module may be further simplified by selecting a single optical component for both the switching and the optical modulation functions.